1. Field of the Invention
The present invention relates to an optical member showing an antireflection performance and a method of producing the optical member. More specifically, the present invention relates to an optical member suitable for stably showing a high antireflection performance for a long time in the visible to infrared region and relates to a method of producing the member.
2. Description of the Related Art
It is known that an antireflection structure having a fine periodic structure of not larger than the visible light wavelength shows an excellent antireflection performance in a wide wavelength region when the fine periodic structure has an appropriate pitch and height. As a method for forming a fine periodic structure, for example, coating of a film in which fine particles having a particle diameter not larger than the wavelength are dispersed is known. In particular, it is known that a textured structure of boehmite that is aluminum oxide grown on a base material shows a high antireflection effect. The textured structure of boehmite is obtained by water vapor treatment or hot water dipping treatment of a film of aluminum oxide formed by, for example, a liquid phase process (sol-gel process) (see Non-Patent Literature: K. Tadanaga, N. Katata, and T. Minami, “Super-Water-Repellent Al2O3 Coating Films with High Transparency”, J. Am. Ceram. Soc., 80(4), 1040-42, 1997). Unfortunately, in this process, the glass substrate may be damaged during exposure to water vapor or hot water.
It has been found that damage of a glass substrate by moisture or vapor can be prevented by disposing a layer of a solvent-soluble polyimide between the substrate and the textured structure of boehmite of aluminum oxide (see Japanese Patent Laid-Open No. 2008-233880). However, the layer of polyimide is low in affinity and adhesiveness to other layers including the substrate and is therefore difficult to be directly used (see Japanese Patent Laid-Open No. 61-171762). In particular, in the case where a textured structure of aluminum oxide is formed through hot-water immersion treatment, detachment between the polyimide layer and its adjacent layer or cracking in the polyimide layer itself or its adjacent layer may occur due to strain stress during formation of the textured.
In the aluminum oxide layer having a textured structure formed by a method of growing boehmite on a substrate, the method is simple and has a high productivity, and the layer shows excellent optical performance. However, erosion of the substrate or elution of the substrate component such as an alkali ion easily occurs during formation of the textured structure by immersion in hot water. Accordingly, a multilayer structure where a thin film is formed between the textured structure of boehmite and the substrate is employed for increasing the antireflection performance while preventing substrate damage. However, the multiplayer structure generates strain stress during formation of the textured structure to cause detachment between each layers or cracking.